The International Telecommunications Union—Telecommunication (ITU-T) G.7712 standard mandates packet 1+1 protection using a pair of associated label-switched paths (LSPs). According to the G.7712 standard, identification of duplicate packets is performed using a sequence number N. The sequence number N is carried in every packet as the first four bytes in the SHIM header for each LSP providing packet 1+1 protection. In order to solve the wrap-around problem encountered upon reaching the 2N upper limit of the sequence number, the ITU-T G.7712 standard specifies use of a sliding window (SW) configured to half of the 2N upper limit of the sequence number (i.e., SW=2N−1).
In other words, the ITU-T G.7712 standard recommends use of a sliding window to solve the problem of losing packets on a leading LSP as the leading LSP sequence number approaches the wrap-around point. The ITU-T G.7712 standard additionally recommends the use of a delay window (DW). As such, the ITU-T G.7712 standard specifies that the number of bits used for sequence number N must conform to a particular set of rules. Furthermore, the ITU-T G.7712 standard recommends that SW size and DW size be configured such that the SW size is equal to the DW size. Unfortunately, the ITU-T G.7712 standard is unclear as to whether the recommendations for SW size and DW size, define (1) in practice experienced (C1) conditions, or (2) a priory required (C2) conditions.
The illustrating examples of the ITU-T G.7712 standard suggest values of SW˜2N−1 and DW=SW−1, such that SW+DW<2N. In general, the suggested values are sufficiently large to allow the C1 interpretation of the ITU-T G.7712 standard (i.e., in practice experienced values) such that, in practice, the maximum number of consecutive packets that can be lost on an LSP and the maximum number of packets the trailing LSP can fall behind the leading LSP never exceed 2N−1. Unfortunately, however, a large sliding window may have a negative impact on the buffer resources, as well as the behavior, of the transport layer (e.g., TCP).
In general, applications requiring high reliability cannot tolerate losses of large numbers of packets. Furthermore, the C2 interpretation of the ITU-T G.7712 standard (i.e., a priory required values) results in numerous problems. Since the DW is generally dependent on the LSP pair (i.e., it may vary over time for a given LSP pair), there is an issue as to handling of a situation in which the required value is exceeded. A related problem is that the SW must be both large enough to cover all variations of the DW, and larger than the maximum number of consecutive packets a working LSP can lose.
In each case, there is an issue as to handling of the various situations in which a required value is exceeded. As such, since a portion of rules of the ITU-T G.7712 standard are violated, packets begin to be irreversibly lost, the associated LSP pair is dropped, and the transport layer (and possibly the application) needs to recover. Although such recovery processing may be performed, restoration time is seriously impacted. Furthermore, both the C1 and C2 interpretations of the ITU-T G.7712 show substantial limitations of the traditional packet 1+1 protection described in accordance with the ITU-T G.7712 standard.